Decreases in the weight, thickness, and size of electronic parts are being promoted. In particular, miniaturization and densification of semiconductor packages (referred to below as packages) are being pushed with the development of multifunctionality of electronic parts. As a result, there is an upward trend in the amount of heat generated by packages such as BGAs. For this reason, there is a desire to further increase the ability of packages to dissipate heat.
Heat dissipating members which transmit heat to the exterior of packages such as heat sinks or radiation fins made of Al are widely used in order to improve the heat dissipating ability of packages. A heat dissipating member and a package are joined to each other by applying grease between them or by attaching the heat dissipating member to the package through a heat dissipating sheet or seal or the like. Joining by these means is inferior to metallic joining with respect to heat resistance, bonding strength, and adhesion. A heat dissipating member and a package are preferably metallically joined to each other with solder or the like.
In recent years, the use of Pb-containing solders (such as Sn—Pb eutectic solder) has been regulated in order to prevent environmental pollution. As a result, there is a trend for solders used for bumps of packages to be replaced by lead-free solders which do not contain lead. If a package which uses a lead-free solder for bump formation can be joined to the above-described heat dissipating member by reflow heating with a reflow profile for the lead-free solder while using flux, soldering of the heat dissipating member and soldering of the solder bumps of the package can be done simultaneously. As a result, in a mounting process, joining by soldering can be achieved by applying a thermal load only once, and the occurrence of manufacturing problems due to a thermal load such as warping of a package or remelting of bumps is minimized.
Indium (In) has a low melting point of 156° C. and good thermal conductivity, and it poses little threat of environmental pollution. Therefore, Patent Documents 1-3 disclose joining of two members to each other using indium.
A lead-free solder alloy made of indium (having an In content of substantially 100 mass %) has excellent ductility. If a small piece of a lead-free solder alloy made of indium is disposed between a heat dissipating member and a package in order to perform soldering, it is expected that the resulting solder joint can easily conform to surface irregularities formed in the joining surface by a thermal load, thereby making it possible to avoid peeling of the joined member which unavoidably occurs when joining with a resin or the like.
However, a lead-free solder alloy made of In has the problems that (a) it is so flexible that it is difficult to form it into a desired shape by punching to form a small piece or by rolling, and (b) in order to guarantee impact resistance, it is necessary for a small piece to have a thickness of 1.5-2.5 mm. This thickness makes it difficult to decrease the size and thickness of electronic parts, and at the time of soldering, the molten lead-free solder alloy oozes to outside of the joint, thereby adversely affecting the insulating properties of the package and causing a short circuit.
It is conceivable to perform soldering with this lead-free solder alloy by, for example, (i) applying a flux to the surface of a metal substrate of a material such as Cu, Ni, or Au having good thermal conductivity, (ii) forming a lead-free solder layer on the surface of the metal substrate by the molten solder plating method (the hot dip plating method) in which the substrate is dipped in molten solder to obtain a connecting member, and (iii) disposing the connecting member between a heat dissipating member and a package and performing reflow heating in the presence of a flux.